String Extraction Where Two Letters Exist

I have a list of 3000 strings, all in seperate cells, in a column.

Within each string, are ( say 13 ) letters ( for convience) "A" through to "M" which are of interest to me

The 'letters' of interest occur either once or twice , or none within the individual strings. When a 'letter' of interest occurs within a string , putting a marker in a column on the same row is sufficient.

Using vba, I have extracted the cases where 2 'letters' exist within the strings, by using 3 nested FOR NEXT loops, with an IF test in the middle of


If (InStr(Cells(i, 2).Value, IndexLetter#1) > 0) and (InStr(Cells(i, 2).Value, IndexLetter#2) > 0)Then
and then put my marker in cells(i,3)

where : IndexLetter#1 is set in the outermost of the FOR NEXT loops as one of the 'letters' of interest, and similarly IndexLetter#2 is set on the next inner of the FOR NEXT loop . The inner most FOR NEXT loop ( i ) is the list of 3000 strings

I want to put a marker next to the string , when only ONE of the 'letters' is present in the string AND NONE OF THE OTHERS